Efficient donor–acceptor type polymer semiconductors with well-balanced energy levels and enhanced open circuit voltage properties for use in organic photovoltaics

Abstract

Three donor–acceptor type copolymers, poly[9,9′-dioctylfluorene-alt-2,5-bis(seleno-2-yl)-2,3-bis(4-hexyloxyphenyl)quinoxaline] (PFSeQ), poly[9-(1′-octylnonylidene)fluorene-alt-2,5-bis(seleno-2-yl)-2,3-bis(4-hexyloxyphenyl)quinoxaline] (PAFSeQ), and poly[N-9′-heptadecanyl-2,7-carbazole-alt-2,5-bis(seleno-2-yl)-2,3-bis(4-hexyloxyphenyl)quinoxaline] (PCSeQ), were synthesized through Suzuki coupling reactions for use as organic photovoltaic materials with high V_OC values. In these polymers, selenophene, which has stronger electron-donating property than thiophene, was adopted as a spacer. The polymers had relatively low optical band gaps (<1.9 eV). We investigated bulk heterojunction type polymer solar cells based on the synthesized polymers used as electron donor materials and [6,6]-phenyl C61 butyric acid methyl ester (PC₆₁BM) or [6,6]-phenyl C71 butyric acid methyl ester (PC₇₁BM) used as the acceptor. The power conversion efficiency (PCE) of the devices was in the range of 1.1–3.3% under AM 1.5G illumination (100 mW cm⁻²). Among these polymers, PFSeQ exhibited the best device performance with a power conversion efficiency (PCE) of 3.3% upon the introduction of PC₇₁BM as the acceptor. This is the highest performance among photovoltaic materials containing selenophene units.